Differential Diagnosis for Hypercalcemia with PTH 1.1 umol/L

Single Most Likely Diagnosis

• Malignancy-associated hypercalcemia: This is often due to the production of parathyroid hormone-related protein (PTHrP) by tumor cells, leading to elevated calcium levels without an increase in PTH. The low PTH level in the context of hypercalcemia strongly suggests a non-PTH mediated cause, with malignancy being a common and critical consideration.

Other Likely Diagnoses

• Vitamin D intoxication: Excessive intake of vitamin D can lead to hypercalcemia due to increased absorption of calcium from the gut. PTH levels may be low or normal as the body attempts to counteract the high calcium levels.
• Hyperthyroidism: Although less common, hyperthyroidism can cause hypercalcemia due to increased bone turnover. PTH levels can be low as the hyperthyroid state can suppress PTH secretion.
• Familial hypocalciuric hypercalcemia (FHH): This is a rare genetic disorder but can present with hypercalcemia and inappropriately normal or slightly elevated PTH levels. However, the calcium levels are usually not as high as in primary hyperparathyroidism, and the PTH level provided is quite low, making this less likely.

Do Not Miss Diagnoses

• Multiple Myeloma: A type of blood cancer that can cause hypercalcemia through the production of osteoclast-activating factors, leading to bone destruction and release of calcium into the blood. Early diagnosis is crucial for treatment and prognosis.
• Primary Hyperparathyroidism with PTH assay variability: Although the PTH level is low, there can be variability in PTH assays, and some cases of primary hyperparathyroidism may present with lower PTH levels, especially if the assay is not sensitive or if there are fluctuations in PTH secretion.

Rare Diagnoses

• Sarcoidosis: A condition characterized by the formation of granulomas in various parts of the body, which can lead to an overproduction of vitamin D, resulting in hypercalcemia. PTH levels are typically low.
• Williams Syndrome: A genetic disorder that can cause hypercalcemia due to sensitivity to vitamin D, leading to increased absorption of calcium.
• Lithium-induced hypercalcemia: Lithium therapy can increase PTH secretion, but in some cases, it may also cause hypercalcemia with low PTH levels, possibly due to its effects on the parathyroid glands or vitamin D metabolism.